Here's why:

* Erosion is the process of wearing away or breaking down rock and soil.

* Deposition is the process of dropping or settling material, often carried by water, wind, or ice.

Stalagmites and stalactites are formed in caves by the deposition of calcium carbonate (CaCO3).

* How it works:

* Water seeps through the cave ceiling, dissolving a small amount of calcium carbonate along the way.

* When the water drips from the ceiling, the calcium carbonate precipitates out and forms a stalactite.

* The water that falls to the cave floor also deposits calcium carbonate, building up a stalagmite.

So, while the initial dissolution of calcium carbonate in the water is technically an erosion process, the formation of the stalagmites and stalactites themselves is a result of deposition.